Using Emacs org-mode for GTD by Charles Cave contains an
introduction into Org-mode, among other things. Charles has also
written a more extensive document (see below), but here we still
link to the earlier version because of its nice overview over
Org-mode.

Org-mode outside Org-mode by Thorsten Jolitz and François Pinard. An
introduction to the available libraries for using Org-mode's concepts and
Org-mode's functionality outside of Org-mode files, e.g. when working with
programming-language source-code files.

Power users describe their setup

In the following documents, the authors describe the details of
their setup, along with the workflows they use to get organized with
Org-mode. While as a beginner you probably should start with only a
simple setup, these descriptions will be a fantastic source of
inspiration once you start growing your use of Org-mode.